Aquagirl was the alias of Marina, daughter of legendary hero Aquaman, and member of the Justice League Unlimited.

Marina wasn't too receptive to the second Batman's enrollment in the JLU to find the supposed traitor within their ranks, though, she was friendlier than fellow teammates Big Barda and Warhawk.
Aquagirl exchanged a few cordial words with Batman, and even invited him for a swim. Batman preferred to pursue his investigation of the team, though.

During the swimming session, Superman, under the influence of Starro, sabotaged the tank and raised the water's temperature. Marina was trapped inside, but Batman blasted the door open with Big Barda's Mega-rod, freeing Aquagirl in the nick of time. Though weakened, Marina quickly recovered. Thereafter, she grew even fonder of Batman.

Shortly after, Aquagirl departed with the JLU to save civilians from a bombardment in Metropolis. She summoned a pod of dolphins and a whale to pick up victims in the water.

When Warhawk was seemingly killed in an explosion, Marina took it quite badly. However, Warhawk soon returned alive, and Superman was revealed to be the instigator. The League confronted their leader, and once Marina touched Superman's chest, a creature emerged. The Man of Steel took on his teammates, but was eventually forced to flee. Aquagirl and others set off after him in the Fortress of Solitude. After subduing Superman with a piece of Kryptonite, Aquagirl tapped into the mind of Starro and ascertained his background. Suddenly, the League was attacked and subdued by Starro's psychic-parasitic clones.
Under the creature's mental hold, Marina tried to open the pool's steel door and release the spawns into the ocean, but she was intercepted by Batman. She was rushed to surface, and relieved of the mind grasp.

After securing every creature, Aquagirl made telepathic contact with Starro, who showed her the whereabouts of its planet. Then, she inserted the coordinates into Big Barda's Mother Box and the League boom-tubed the creatures back home.